Stability of Perovskites : Intrinsic Degradation Factors

Introduction

14

1.2.2 Challenges of Halide Perovskites

Although the PCE of perovskite solar cells has gone from single digits to over 22% in a few years’

research, they still face challenges that prevent them from competing with established technologies.^{23, 88} Therefore, at this stage of their development, the key issues are how to achieve further improvements in efficiency and long-term stability of these materials under device operation. The advancements in materials processing in the past couple of years have led the research community to profoundly investigate intirinsic vs. extrinsic degradation mechanisms.²³

The stability issue is a major hurdle for the commercialization of perovskite optoelectronics.^{64, 89} The degradation factors of perovskites can be divided into two: (i) extrinsic environmental and (ii) intrinsic degradation factors. Before investigating the intrinsic ones in detail in the next chapter, it is important to understand the extrinsic environmental degradation factors that increase degradation pathways, which tend to be irreversible.²³ First, ultraviolet (UV) light, present in the full solar spectrum, is detrimental to the long-term stability of perovskites due to the absorption by the electron-selective contact, initiating a chemical degradation.⁹⁰ Second, analogous to organic PV, perovskites are severely affected by moisture which induces rapid degradation of the perovskite layer in the devices.^91-92 In addition to these factors, elevated temperature⁹³ and oxygen⁹⁴ are also responsible for the instability of the perovskites. These extrinsic factors can be retarded using the sealing technologies industrialised for organic electronics, enabling oxygen and humidity barriers and protection against UV light.⁹⁵

Introduction

15 It is important to keep in mind that only the steady-state values under continuous illumination have practical significance and should take precedence over simple J-V curve scans.^{64, 98-99}

Figure 1.14 J-V response of PSCs at different (A) scan rates¹⁰⁰ and (B) light-soaking conditions¹⁰¹. There are several suggested mechanism for understanding the origin of hysteresis.^102-103 The leading model among them is ion migration associated with a change in interfacial fields and barriers resulting from accumulation of ions at interfaces, which causes charge recombination.^104-105 In the presence of an electric field created by an external voltage bias or light, ions migrate across the bulk of the perovskite layer and reach the external interfaces where they accumulate. The charge collection efficiency of the device is adversely affected where these interfaces act as recombination regions. Instead, if recombination at the interfaces is reduced, then the build-up of photogenerated charge carriers contirbutes to efficient collection of diffusive currents during the forward scan. Therefore, low hysteresis is resulted from the combination of low interfacial recombination and resultant high photogenerated carrier populations at forward bias, despite the presence of ion migration. This model also explains the reduction in hysteresis when the contact materials are changed, while this is unlikely to significantly influence the behaviour of mobile ionic charges within the perovskite phase.¹⁰⁴

Based on this model, the degree of hysteresis is highly dependent on the interface properties and choice of contact materials, which appear to control the interfacial trap density as shown in Figure 1.15.^99,

103-104, 106-107 To suppress the hysteresis, the traps can be passivated by modifying the interface with fullerene derivatives to reduce the nonradiative recombination channels due to the reduction in the trap density at this interface.¹⁰⁶ Additionally, the hysteresis can be alleviated by the fine-tuning of the Fermi level of the contact material and the perovskite. With this, the charge transport is promoted through the contacts, which ultimately decreases the hysteresis, instead of accumulation and recombination of the charges at the interface.⁹⁹ However, the hysteresis of PSCs is still under lively debate and no effective methodology has been discovered yet despite much effort to better understand the origin and minimization of the hysteresis.¹⁰²

Introduction

16

Figure 1.15 Schematic representation of the surface recombination reduction by passivating the trap states.¹⁰⁶

1.3.2 Halide Segregation

It is estimated that a halide perovskite with a band gap of around 1.7-1.85 eV can boost the PCE of commercial PV devices to 30%, where PSCs are used as a top cell in tandem applications. Nevertheless, most of the highest-performing PSCs have band gaps around 1.5-1.6 eV.37, 98, 108 Although the desired band gap can be easily achieved by tuning the halide composition, there has been a unique challenge called halide segregation under illumination in mixed halide perovskites which is detrimental to the PV performance and an obstacle in the path of applications of PSCs in high-efficiency tandem solar cells (Figure 1.16).37, 64, 109-110

Figure 1.16 Schematic representation of halide segregation in perovskites.

When the perovskite is illuminated, the photovoltage induces an additional electric field acting across the perovskite layer, and causing the migration of ions/vacancies.⁶⁴ This results in two phases which are iodide-rich minority and bromide-enriched majority domains, causing the formation of a new peak in PL spectra as well as a splitting in X-ray diffraction (XRD) reflections.¹¹¹ Although the segregation effects are reversible under short-term illumination, the research community has voiced concerns

Introduction

17 regarding the influence of halide ion mobility on the long-term stability and open circuit voltage of PSCs.¹¹² Therefore, several strategies were suggested to overcome the photo-instability of mixed halide systems such as growing larger grain sizes¹¹³ to obtain fewer grain boundaries where ion migration predominantly occurs¹¹⁴ and reduction of the Goldschmidt tolerance factor which improves the stability of perovskite films^115-116.

In addition to the PV applications, lighting devices prepared from perovskites suffer from this phenomenon due to the application of voltage instead of illumination. It has been noticed that the emission from mixed halide perovskite NCs in LEDs red-shifts reversibly during device operation and returns slowly toward the original state after resting. Since the electroluminescence (EL) and PL shifts are completely consistent with each other, these shifts are interpreted as a result of intrinsic changes within the perovskites, e.g. halide segregation. Therefore, it was suggested that band gap tuning of pure (non-mixed) halide systems via quantum size effects might be a more successful way to cover the entire visible spectrum in lighting applications.¹¹⁷